It depends on the size of the tank you have, and how aggressive your Betta is.
When I bought my first three and sat their bag in the water to adjust, Madonna started flaring like crazy, and he's super docile. So we took Madonna out and rearranged the whole tank, then after the Panda Cory's were adjusted properly we released them in first, then re-added Madonna to the tank. We haven't had any issues since. I think he thinks he is one now, he just kind of energetically follows them around.

Quarantining new fish is important too, usually a two week period treatment, especially if they look like they have the beginnings of a disease.

I'm still new to the fish hobby I get a lot of help from my hubby, but I hope this information helped answer your question a little bit.

When I had Bettas before, I put the bag in a cup I had rigged to hang on the side of the tank. Since cups are opaque, the Betta can't see what's in it. Then I turned the lights off to release the tankmates when they were acclimated.